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Getting owned by an 87 TBI

First, thanks in advance for all the helpful information on this forum. I've got an '87 GMC V2500 w/ HD option. It stumbles and bumbles under load. I've googled the hell out of my issues and below is all the new parts and tests I've thrown at it over time. The new distributor was the last new thing I installed. I really thought it was going to fix the problem since the old one looked crusty and rusty, but no such luck. The truck needed all new ignition parts anyway, so no regrets replacing them, but the truck is still undrivable.

Here is my main problem I'd like to solve. Looking for any ideas on where to go next, I'm at a loss at this point.

With MAP sensor plugged in (which is brand new and verified via OBD scanner to be showing proper readings), I'm getting about 15-16in of vacuum at idle. Watching the timing mark as I increase rpms at the throttle and hold them steady, I see timing go through a cycle of dipping down into where it shouldn't be. It dips down to around 8 degrees and then goes back up past the end of the timing mark gauge where it should be...then keeps bouncing back and forth as vacuum bounces up and down, too. Unplug the MAP and vacuum at idle goes to 21 in. As I increase and hold a higher rpm, I see timing advance increase, no more dips as when MAP is plugged in...it seems to be operating properly in terms of what you expect a healthy engine to do as far as timing and vacuum readings are concerned.

-New MAP Sensor (MAP voltage and kpa readings taken from obd1 reader are appropriate/correct)
-New IAC Valve (did the relearn)
-New Distributor (set at 0 degrees w/ advance unplugged), plugs (gapped at .035) & wires.
-Smoke tested, replaced some questionable hoses and capped off non-essential stuff
-Compression tested when I did the plugs and all looked OK.
-New fuel filter
-Went over grounds as best I could
-Fuel pressure (gauge placed between filter and TBI unit) holds steady at 11psi as the truck stumbles and bumbles around the block at various throttle positions.
-Truck has headers and has no O2 sensor (needs a bung welded in)
-EGR solenoid is capped between solenoid and EGR.

Amazon has an acdelco ECM for $105 with a $90 core rebate, so that is in the mail as well. For $15, I figured why not go for it?

What you've described isn't the problem. Well, maybe it is but it really sounds like normal operation. See, when the vacuum drops so does the spark advance. When you first open the throttle the vacuum drops and the ecm reduces timing. Then as the rpm increases, vacuum increases, and the spark advance increases.

How easy is it to increase engine RPM with the MAP disconnected? The engine should be running pretty rich with the main vacuum line disconnected. Have you ever confirmed that the MAP sensor is seeing a full vacuum signal at idle when connected?

Yeah, sorry, I meant the problem I want to solve is why does vacuum / timing advance momentarily but repeatedly drop when I raise rpms above idle and hold it there (while in Park)? My truck does have the usual tan/black wire that I disconnected when setting timing to 0 degrees.

With MAP circuit unplugged (I'm unplugging the connector, not removing the hose and creating a vacuum leak), yes it certainly runs rich, RPMs of course jump up, but vacuum readings look great and continue to look as expected if raising and holding the RPMs in park.

Map sensor is plugged into correct port on back of TBI between fuel lines. I've tee'd the line between TBI and MAP and used a vacuum gauge to confirm the readings I shared earlier.

I thought that even without an o2 sensor, my truck should run strong as far as not stumbling and holding a consistent vacuum / timing advance for above idle RPMs while in park. It will just always remain in open loop and therefore may not run as optimally as when reaching closed loop, but it should still run relatively strong assuming all other components are working as they should. Is that not correct?

You are correct in expecting the truck to run well enough to drive with the O2 sensor unplugged. The older calibrations were not always as good at running their best without O2. Is the ECM setting a code without the sensor?

I would expect the truck to stumble and run poorly without the MAP connected. I would not expect the rpm to increase and the truck to run well enough to continue testing.

MAP sensors can respond very quickly. Sometimes faster than a vacuum gauge and definitely faster than the ALDL reader can show. I have an oscilliscope that I use to watch sensor signals when needed. It's possible that the MAP reading is changing due to a loose connector or loose connector pins or due to a broken or chafed wire. You could try pinching off the vacuum signal to the MAP while the engine is running to trap vacuum and force the sensor to have the same reading, then increase RPM to see if the timing bounces around. You could also turn the key on and leave the engine off and try gently moving wires while watching voltage on the MAP signal line. I often found the old scantool readings hard to use because they updated so slowly. I think that a digital voltmeter is a better tool for finding intermittent problems.

-Video starts with engine idling @ ~950rpm (i know that needs addressed) w/ map sensor and vacuum hose all hooked up.
-@ :06 I hit the throttle and hold it steady, engine starts seeking
-@ 1:04 I unplug the map sensor, rpms go up to ~1500 and vacuum to around 22
-after that I blip the throttle a couple times as well as increase rpms up to 3000 towards the end and hold it steady for a bit.

What happens if you move the TPS without moving the throttle plate? It should cause the injectors to deliver additional fuel. Does the engine run better then? You might have to remove the TPS from the throttle body or use a spare TPS to do this.

Problems underload would always lead me to put a pressure port adapter in the o2 bung, just to check the backpressure of the exhaust system. I don't know if you have a cat or not, but I have seen dual wall pipes collapsed too, but its not as common. I use my vacuum gauge as its reads a small amount of pressure. The adapter fitting can be made or the tool trucks have them.

Cats are long gone on this truck. I'll try the TPS idea next when I have time.

I guess one thing I could use some opinions on is what that video I posted proves. Could a vacuum leak still be a problem? What else could be a culprit - ECM, injectors, ????

It could be almost anything. You are there, we get to look at videos and read data that you interpreted. The gauge was blurry but I think it shows you have good vacuum at idle and it does drop when you open the throttle. Sometimes when I was stuck on a car I would recommend fixing what you know is wrong. With all that's disconnected you had better be throwing a handful of codes. If not then thats a big problem. Hook it all backup, clear memory and see if no codes show up. The factory gm manuals are a very good start. They have no code drivability charts, a lot of them end with "replace with known good.... and retest. At least it gives you a few Ideas before throwing your money at it. In your case you are not doing yourself any favors with the o2 sensor not hooked up. Will it fix the problem? Who knows but at least you will get to see if the fuel mixture goes rich or lean during its problem and the computer will get that info too. With headers, unless they are shorty with an o2 bung, you probably need a three wire heated o2 anyway.

I guess one thing I could use some opinions on is what that video I posted proves. Could a vacuum leak still be a problem? What else could be a culprit - ECM, injectors, ????

Your video appears to show vacuum readings changing along with engine rpm. To me it proves that your initial description isn't too far from the symptoms on video. The engine could be lean or rich so I'm suggesting you to enrichen the mixture. If it runs better the solution may be to find out why the engine's lean. That would also tie in with why the engine appears to run better if the MAP is unplugged.

This is a tough way to diagnose because you're filtering what you're telling us. Your initial description is "stumbles and bumbles under load." A stumble is a specific condition relating to how the engine runs. A bumble is a cartoon character. All we can do is guess at that you're trying to describe. Then you describe a condition that you'd like to solve. But the description sounds like mostly normal operation. Except you see a problem with it. We're left guessing again. And then you post a video as if it's conclusive. We haven't been there watching all the steps you've gone through in this fight. So your video shows that vacuum fluctuates and the MAP sensor readings fluctuate as well which again seems like normal operation.

Here's the deal. If you're only trying to gain insight about how the engine and ecm work together, then we can help. The ecm will vary timing based on load, rpm, coolant temperature, and knock signal. More rpm means more advance. More vacuum means less load means more advance. Colder engines get more advance. Knocking engines get less.

If you want assistance diagnosing the problem we can help with that also. But you need to help us understand what's happening because we aren't there with you. How did the problem begin? Did it come on suddenly? Are there times when it's better or worse? What does the underhood sticker say about setting timing? Is the engine stock? You mention a vacuum leak... did you have one previously? What did you do to fix it?

Sorry, either I'm totally missing something or I didn't explain what's happening in the video clearly. What I intended was to ask if I can focus on investigating a (hopefully) short list of specific things given the behavior and readings in the video in combination with items I've already done listed in my 1st post, like dist, plugs, wires, timing at 0, etc.

After 1:04 in the video, when the MAP is unplugged, my intention was to show via the vacuum gauge that the engine seems to be mechanically healthy per the classic tests of how the gauge's needle should react under various conditions. By the way, I do see vacuum increase a bit as steady rpms increase, which I believe indicates exhaust is flowing well.

At the start of the video, with the MAP plugged in and feeding the ECM, I intended the video to show, first, that the MAP sensor voltage readings look correct. Second, between :05 and :024, I am not manually blipping the throttle open and closed. I am doing my best to hold the throttle open at roughly 2000 - 2100 RPMs. This is my "stumble and bumble" :). It does the same thing under load in gear. In the video, RPMs are cycling/dropping from ~2100 to ~600-700. I don't think I can do an aggregated knock count with my scanner, but it never reports a knock. Coolant temp is being reported correctly via OBD.

So what I was hoping you could help me learn is what the ECM is trying to do here. I learned it uses RPM and MAP output to set timing. With MAP unplugged, it uses a default fuel map. RPM and MAP readings look correct via OBD scanner.

This is a new-to-me-truck - I have no standard or history to compare how it's running with. Engine might not be stock. When it first got delivered, timing was set at 8 degrees advanced, maybe even a little more. I guess technically it ran better than it does now, but still not really good at all, and idle RPM was so high it was a pretty brave move to actually put it in gear.

One tough aspect about diagnosing problems in a responsive system is trying to separate problems caused by the response from problems causing the response. I believe some of what you are looking at are problems caused by the response. The timing will vary as MAP varies. This can cause a cycle of hunting rpm. Timing further varies with RPM which can widen the range of the response. There's a chance that the timing is not the cause of the problem.

I am an "old fashioned" mechanic and sometimes I prefer to reduce the computer's ability to affect the engine and reduce complication. If I suspected the timing variations were masking a problem or causing further problems I would disconnect the tan/black wire to lock the timing in a fixed position. That would allow me to use a timing light to set the distributor where I want at a particular RPM then focus on the rest of the engine. I have a dial type timing light so I can read the timing setting well past the timing tab. I would set timing to an appropriate value to see if the engine runs smoothly and provides a steady vacuum reading.

Disconnecting the MAP sensor is not usually one of my options. With both fuel and spark controlled by MAP it really changes the computer's response. The fuel calculation is based on throttle position, coolant temp, and the VE map. It can be way off compared to the reality.

One issue that you can check is the base timing setting. On many of the early trucks it was six degrees. Setting timing six degrees retarded would drastically alter how the engine ran.

With tan/black wire disconnected, max engine vac was achieved around 10-12 degrees advanced. But, per info on the truck's shroud, base timing should be set at 0 degrees.

Double checked the injector spray pattern - nice cone shape with no big drops. I see the amount of spray cycle up and down with the rpm drops shown in my video.

One thing I wanted to ask is if an o2 code should be triggered while in Park or if it needs to be in gear. With no o2 sensor plugged into the connector, I think I should expect code 13 to be triggered under these circumstances:

Other thing to add is that when the rpms start dropping and cycling back up when I hold the throttle at higher rpms, live data seems to show that IAC isn't moving, so I guess it's the timing getting retarded that's more responsible for the rpm drop.